Glasses are obtained via sol-gel process. Glass-forming ability of a system is regarded as closely related with the gelation ability, which is reflected in its thermal stability. In the present study basic systems such as B_2O_3-Na_2O, B_2O_3Na_2O-Al_2O_3 (TiO_2) and SiO_2-Na_2O systems were chosen to study gel regions, thermal stability and glass-forming ability. First, variations of gel regions were investigated with the addition of water or catalysts (hydrochloric acid or ammonium). Gel regions were at B_2O_3 <100 mol% and>60 mol% in the B_2O_3-Na_2O system. Gel regions emerged in Al_2O_3 rich compositions in the B_2O_3-Na_2O-Al_2O_3 sys- tem, and gel region was at SiO_2=60-100 mol% in the SiO_2-Na_2O system. Little effect was observed with water of ammonium addition, but with HCl addition crystals precipitated in most compositions except SiO_2=100 mol% composition. Gelation time was reduced with the addition of water or NH_4OH. DTA measurements were conducted on gels with a heating rate 5ﾟC/min to determine the crystallization temperature Tc. Liquidus temperature Tl was determined by DTA method or by direct observation with a hot-stage microscope. Tc/Tl ratio was assumed to represent the thermal stability of a gel or a glass. In the Tc/Tl-composition curvea maximum and a minimum were observed at B_2O_3= 70 and 80 mol%, respectively, in the B_2O_3-Na_2O system. Similar Tc/Tl dependency was identified for melt-quenched glasses. It was proved that Tc/Tl ratio indicates the gelation ability or the glass-forming ability of a system. Tc/Tl - Q* (critical cooling rate for glass formation) relationship was established among the experimental data. Computer simulations were made to confirm this relationship.